Stepper motors have been used to drive analog gauge pointers, particularly in motor vehicle instrument clusters. Normal movement of the pointer is typically accomplished by micro-stepping the stepper motor, and the controller determines the relative pointer position by maintaining a step count. This eliminates the need for a position sensor, but requires a known initial position of the pointer. Since the pointer can be off-zero at power up, a return-to-zero half-step sequence is utilized at power-up to establish an initial zero position of the pointer. A typical return-to-zero step sequence involves driving the stepper motor through a specified angle of rotation in order to move the pointer against a fixed stop. Unfortunately, this can produce perceptible flutter of the pointer, and even audible noise, because certain steps of the return-to-zero sequence produce off-zero movement of a pointer that has already returned to the zero position. This phenomenon is described in some detail in the U.S. Pat. No. 5,665,897 to Lippmann et al., assigned to the assignee of the present invention, and incorporated herein by reference.
One way of addressing the pointer flutter issue is to simply deactivate the motor windings during the steps that might produce off-zero pointer movement. While such an approach can be simple to implement, the torque generated by the motor may be insufficient to reliably return the pointer to the rest position under certain conditions, and substantial errors can occur in gauges where the motor lacks a geartrain between its rotor and output shaft. The aforementioned Lippmann et al. patent discloses a reliable but more sophisticated approach involving a factory calibration learning procedure and a wake-up routine executed periodically during ignition off periods. What is needed is an improved return-to-zero control method that is both simple and reliable.